Safari Update
Software update for Safari has been running for hours looking for updates What is the problem
iMac 21.5″, macOS 11.5
I understand Software Update is "Checking for updates..." with no end in sight.
Several workarounds have been proposed, such as restarting your Mac in "Safe Mode" and checking for updates while in that mode, and / or resetting NVRAM. Try both.
How to use safe mode on your Mac - Apple Support
Reset NVRAM on your Mac - Apple Support
Related: Software Update stuck at "Checking for updates"… - Apple Community